We are using serial sections, serial tilts, and electron microscopy to track the microtubules (MTs) of the mitotic spindle in 3-D. The trajectories of the spindle MTs that connect with chromosomes have been determined, together with those of the other MTs that come close to them (McDonald et al., 1992). We have also reconstructed the non-kinechore or "inter-polar" MTs (ipMTs) of PtK cells in metaphase and anaphase (Mastronarde et al., 1993). Our findings demonstrate that non-kinetochore ipMTs interdigitate to a greater extend than had previously been appreciated in both metaphase and early anaphase. Further, the minus end of many of these MTs is farther from the pole than is generally thought. During early anaphase, before spindle elongation begins, MT elongation is already underway. Subsequently in anaphase, the plus ends of the ipMTs appear to slide relative to one another to enhance spindle elongation, but the minus ends of these MTs move little, so sliding of ipMTs is not a major mechanism in anaphase B in mammals. ipMTs have a marked tendency to end in the kinetochore bundle, as previously defined. These ends usually lie very near kintochore MTs, suggesting an interaction. We are now extending this investigation by using axial tomography to reconstruct the details of MT interactions with kinetochores, centrosomes, and with each other.